This invention relates to Digital Subscriber Line (DSL) modems for communicating over telephone subscriber loops, and more particularly, to optimizing transceiver training for Time Compression Multiplexing DSL (TCM-DSL) modems.
The demand for modem transmission speed continues to soar as more telephone customers in more countries send more data traffic over phone lines. While it is feasible to run high-speed fiber-optic cable to some new customers, existing customers are connected to the phone system by slower copper wires such as untwisted or twisted-pair lines. The cost of replacing all existing copper wires with higher-speed fiber-optic cable is prohibitive. Thus, higher-bandwidth technologies that use the existing copper-cable phone lines are desirable.
Basic rate Integrated Services Digital Network (ISDN) boosted data rates over existing copper phone lines to 128 kbps. Special termination and conditioning of the existing copper phone lines is required for ISDN.
DSL modems are now becoming available. Several variations of DSL technology (referred to generically as xDSL) are being explored, such as High Bit Rate DSL (HDSL), Rate Adaptive DSL (RADSL), Very High Bit Rate DSL (VDSL), and Asymmetric DSL (ADSL). ADSL is particularly attractive for consumer Internet applications where most of the data traffic is downloaded to the customer. Upstream bandwidth for uploading data can be reduced to increase downstream bandwidth since most Internet traffic is downstream traffic. ADSL provides a bandwidth of up to 8 Mbps in the downstream direction, or up to 2 Mbps if symmetric DSL is used. See, for example, U.S. Pat. Nos. 5,461,616, 5,534,912, and 5,410,343 for descriptions of ADSL technology.
Cross-talk Using Pulp Cables Limits xDSL
The wider bandwidth required for xDSL transmission creates higher cross-talk interference among copper pairs in the same cable-binder group. The level of the cross-talk varies for different cable structures and materials. In particular, some countries such as Japan and Korea use telephone cables with a paper-based xe2x80x9cpulpxe2x80x9d insulator rather than the plastic-insulated cables (PIC) used in the United States. These pulp cables produce much more cross-talk interference than the PIC cables. Thus, it is more difficult to deploy wide-band xDSL services in those countries since their existing telephone cables are prone to cross-talk interference.
FIG. 1 shows the problem of interference from existing ISDN lines. Central Office 8 (CO) contains several ISDN line cards 14 that connect the telephone network backbone to local lines 20 that are strung to the customer premises equipment (CPE). Remote ISDN terminal adapters or modems 12 are located at different remote customer sites within a few kilometers of central office 8.
Local lines from ISDN line cards 14 to remote ISDN modems 12 are usually routed through one or more cable bundles 18. These telephone-cable bundles 18 may contain dozens or more separate telephone lines or copper pairs. Standard voice services, ISDN services, and newer xDSL services often must share the same cable bundle. Since lines run close to other lines in cable bundles 18 for long distances, mutual inductances can create cross-talk interference or noise on lines 20.
For voice services such as Plain-Old-Telephone Service (POTS), the frequencies are low, so interference is negligible. ISDN digital services use a higher bandwidth of around 80 to 320 kHz. Interference begins to cause problems at ISDN frequencies. New xDSL services usually use even higher bandwidths. For example, ADSL bandwidths are usually above 1 MHz and have significant cross-talk problems. Cross-talk from other digital services such as older ISDN and T1 in a cable bundle can severely restrict xDSL speeds.
Due to different cross-talk interference characteristics, different line codes are used for basic rate ISDN. In countries such as the U.S., where better-insulated PIC cables are used, full-duplex data transmission with echo cancellation is deployed. Echo cancellation by the receivers removes the echo by the locally-transmitted signal so that the remotely transmitted signal can be received. Thus, both ends of the line can transmit simultaneously. Full-duplex data transmission with echo cancellation is described in International Telecommunication Union-Telecommunication Standardization sector (ITU-T) G.961, Appendix II, or T1.601, which is incorporated by reference herein in its entirety.
Japan Uses Half-Duplex ISDN
In countries such as Japan, where the noisy pulp cables are installed, a different ISDN system is often deployed. To eliminate the near-end cross-talk (NEXT) interference, TCM-ISDN is used rather than echo cancellation full-duplex. In such a system, the ISDN line cards at the CO are synchronized so that they all transmit at the same time. The ISDN line cards all receive during a different time period. Thus, NEXT interference during reception is eliminated since none of the other ISDN modems at the same side are transmitting during the reception time period. Although far-end-cross-talk (FEXT) interference exists, it is usually much weaker than NEXT. TCM-ISDN service is described in ITU-T G.961, Appendix III, which is incorporated by reference herein in its entirety.
FIG. 2 is a timing diagram for a TCM-ISDN line. During time period or window 22, data is output from the CO to the remote ISDN modem at the customer premises. This data arrives at the remote modem after a delay, during reception window 24. The customer premises ISDN equipment uses a burst clock detector to determine the timing of the receive downstream burst and to generate the timing for its transmit upstream burst. A pause occurs when no data is transmitted. This pause is sometimes called the turnaround period. During period 26, upstream data is transmitted from the remote modem to the CO, which arrives at the CO after a delay, during window 28.
At any particular time, only one end of the TCM-ISDN line is transmitting, while the other end is receiving. Echo cancellation is not needed since the echo of the transmitted signal does not have to be removed. Since each side transmits in slightly less than half of the time, the data rate during transmission has to be approximately doubled to obtain the same average data rate. This translates to a higher frequency bandwidth, which in turn creates more cross-talk. While such a TCM-ISDN system is effective for reducing cross-talk in the TCM-ISDN system itself, it is difficult to add newer xDSL systems in the same cable bundle because of the cross-talk from the ISDN lines.
Synchronized ISDN Lines Create Interference for xDSL
Newer xDSL services, such as HDSL and ADSL, use full-duplex transmission based on frequency-division-multiplexing (FDM) or echo cancellation. Therefore, the receiver at either side receives all the time. If such an xDSL modem is installed in the same cable bundle as the TCM-ISDN, the strong NEXT during the transmission time for the same-side TCM-ISDN modems will severely affect the reception of the xDSL signal.
FIG. 3 is a diagram of interference at a CO from several ISDN lines transmitting in synchronization. During transmit window 22, a burst of data is sent from the CO to the remote sites. NEXT is particularly strong during transmit window 22, since the ISDN devices at the CO are all transmitting. During receive time window 28, these ISDN devices at the CO are not transmitting. Interference is primarily FEXT, which is weaker than NEXT since it is attenuated by the length of the telephone line.
TCM-ISDN Transmitters Often Poorly Filteredxe2x80x94FIG. 4
FIG. 4 is a transmitting-signal spectrum of a TCM-ISDN modem. For background information on a TCM-ISDN telephone system, see U.S. Pat. No. 5,265,088 by Takigawa et al., and assigned to Fujitsu Ltd. and Nippon Telegraph and Telephone Corp. (NTT). This coding scheme uses Pulse Amplitude Modulation (PAM) with alternate-mark inversion (AMI). In this scheme, a binary zero is represented by no pulse, and a binary one by a positive or a negative pulse. Each symbol carries only one bit. The ISDN lines are designed to operate over a frequency range of zero (Direct Current or D.C.) to about 320 kHz. Since ISDN operates down to 0 Hz, no lower band is available for POTS voice calls.
ISDN signals decay slowly above 320 kHz. The higher harmonics are not necessary to carry information, but they are often not filtered by the transmitter to a low level, resulting in long, high frequency tail. When ISDN was first deployed, the upper frequencies were not used by other devices, so interference in the higher bands was not a problem. However, it is a severe problem for newer xDSL services that use the higher frequency band.
ISDN Interferes With xDSLxe2x80x94FIG. 5A
Interference from ISDN is generated in these frequency bands used by ADSL and other forms of xDSL, and vice versa. Lower-quality cables such as pulp cables do not sufficiently insulate ISDN lines from ADSL lines.
Full-duplex xDSL Data Transmissionxe2x80x94FIG. 15
Annex C of ITU-T recommendations G.992.1 and G.992.2 (hereinafter also referred to as xe2x80x9cAnnex Cxe2x80x9d), which is incorporated by reference herein in its entirety, defines specific requirements for TCM-DSL modems operating in TCM-ISDN environments. More particularly, Annex C defines a Dual Bitmap (DBM) encoding method for providing dual bitmaps that are switched synchronized with the burst cycle of TCM-ISDN to provide a data stream having dual bit rates. The method is based on the observation that for short local loops (e.g., less than about two kilometers), the channel signal-to-noise ratio (SNR) can be sufficiently high during NEXT interference to transmit data at a low bit rate. Thus, under certain conditions DBM allows full-duplex operation of TCM-DSL modems by employing different bit rates under NEXT and FEXT interference, respectively.
Referring to FIG. 15, there is shown a diagram illustrating full-duplex xDSL data transmission using DBM encoding. During upstream (US) NEXT time 1500 (which is downstream (DS) FEXT time), a CO modem 47 (FIG. 9) transmits data at a downstream bit rate 1504 and receives data at an upstream bit rate 1508, wherein the upstream bit rate 1508 may be significantly lower than the upstream bit rate 1510. During this same time period, a CPE modem 48 (FIG. 9) transmits data at the upstream bit rate 1508 and receives data at the downstream bit rate 1504. During US FEXT time 1502 (DS NEXT time), the CO modem 47 transmits data at the downstream bit rate 1506 and receives data at the upstream bit rate 1510, wherein the downstream bit rate 1506 may be significantly lower than the downstream bit rate 1504. During this same time period, the CPE modem 48 transmits data at the upstream bit rate 1510 and receives data at the downstream bit rate 1506.
The DBM encoding method described above typically works for short local loops (e.g., less than about two km). For longer local loops, however, the SNR during NEXT time (1500 for US, 1502 for DS) is typically too low for modems to send any data. In that case, the data transmission occurs only in FEXT time (1500 for DS, 1502 for US). This is referred to as xe2x80x9cSingle Bitmapxe2x80x9d encoding (SBM), which is a special case of DBM encoding. With SBM, the CO and CPE modems 47, 48, are transmitting only in FEXT time 1500 and 1502, respectively, and do not transmit simultaneously (i.e., half-duplex mode).
In DBM encoding, bit rates can be changed by changing the bitmaps used to encode the symbols to be transmitted. As is understood by those skilled in the art, a xe2x80x9cbitmapxe2x80x9d determines the number of bits which can be encoded into a symbol. A xe2x80x9csymbolxe2x80x9d is the basic unit of information transmitted by the modem. The number of bits encoded into each symbol is limited by the quality of the communication channel. The quality of the communication channel can be represented by its SNR. Thus, a system employing DBM includes at least two bit maps for providing different data rates for NEXT time and FEXT time, respectively.
In order for modems to transmit and receive data, it is important that the modems are properly initialized. According to ITU-T draft recommendations G.992.1 and G.992.2 (October, 1998), the initialization includes the following four phases: (1) initial handshaking, (2) transceiver training, (3) channel measurement, and (4) message exchange. The initial handshaking phase is defined in ITU-T recommendation G.994, which is incorporated by reference herein in its entirety.
In the current version of G.992.1 and G.992.2, the transceiver training is done in half-duplex mode, i.e., all the training signals are sent only in FEXT time (1500 for DS, 1502 for US). At the end of the transceiver training, the CO modem 47 informs the CPE modem 48 whether DBM or SBM will be used. Depending on the mode (DBM or SBM), the transceiver training could be optimized for the selected mode.
For DBM operation, the CO and CPE modems 47,48 transmit continuously in full-duplex mode. If operating in full-duplex mode, an echo signal from the CPE modem 48 transmitter is added to a remote signal received from the CO modem 47. This combined signal is often stronger than the remote signal by itself. To accommodate both signals, the CPE modem 48 receiver adjusts its gain and dynamic range. Moreover, the CPE modem 48 receiver may employ an echo canceller or echo filter to remove the unwanted echo signal generated by the CPE modem 48 transmitter.
By contrast, when operating in the SBM mode the echo signal and the remote signal are not received by the CPE modem 48 at the same time. At any particular time, one end of the TCM-ISDN line is transmitting, while the other end is receiving (e.g., half-duplex). Therefore, no gain adjustment due to echo or echo canceller is needed while operating in the SBM mode.
A problem, however, arises if the CPE modem 48 trains with SBM encoded signals, then subsequently transmits xDSL data using DBM, and vice-versa. To properly train TCM-DSL modems for full-duplex communication, the training signals should be full-duplex. Similarly, to properly train TCM-DSL modems for half-duplex communication, the training signals should be half-duplex. Thus, to train a TCM-DSL modem properly requires knowledge of the mode of communication (e.g., SBM or DBM) prior to training. Unfortunately, existing systems do not provide such information, as described in detail below.
During the initialization of such a CPE modem, the CPE modem does not know if it will be using SBM or DBM until after transceiver training has completed. Therefore, its receiver must allocate additional dynamic range for the echo signal in case DBM is selected. This added dynamic range is wasted, however, if SBM is selected instead of DBM. Further, since the echo signal and the remote signal are typically not received at the same time during transceiver training, the dynamic range of the CPE modem receiver cannot be preset to accommodate the combined signal. Thus, if the training signal used in SBM is half-duplex, the transceiver training will not be optimized for full-duplex data transmission in DBM, and vice versa.
Additionally, to remove echo from the remote signal during DBM operation, a different frequency band is allocated for upstream and downstream communication which is typically achieved with either echo filters or echo cancellers. Echo filters typically employ a guard band to prevent cross-talk interference. Unfortunately, the guard band reduces the available frequency bandwidth for data transmission. Echo cancellers do not need a guard band but are more complicated to implement.
Neither echo filters nor echo cancellers are needed for SBM mode. Hence, the TCM-DSL system configuration should be optimized based on the mode selected (DBM/SBM) to provide a wider bandwidth for data communication due to the elimination of echo filters and associated guard bands.
Accordingly, there is a need for an xDSL system that can be added to an existing telephone cable system where TCM-ISDN services co-exist with new xDSL services in a common cable bundle. It is desired that a DBM encoding scheme be incorporated in the xDSL system in a manner that allows TCM-DSL modems to optimize for full-duplex or half-duplex communication during transceiver training.
The present invention is directed to improving transceiver training in TCM-DSL modems operating under NEXT and FEXT interference. Current ITU-T draft recommendations (October, 1998) provide for dual bitmaps that are switched synchronized with the burst cycle of TCM-ISDN to provide a data stream having dual bit rates. These recommendations also provide for a single bitmap mode having a single bit rate only in FEXT time with the bitmap for NEXT time turned off. With systems and methods defined in ITU-T draft recommendations (October, 1998), a modem does not know which mode has been selected prior to transceiver training, which results in less than optimal transceiver training and modem configuration. In the present invention, mode selection is exchanged between modems prior to transceiver training (e.g., during initial handshaking). By exchanging the mode selection prior to transceiver training, the transceiver training is improved and the modems are configured properly for the type of data transmission (e.g., full-duplex, half-duplex).
A method of training modems for DSL systems under TCM-ISDN interference comprises the steps of: exchanging between a first modem and a second modem a mode selection prior to transceiver training; providing full-duplex communication between the first and second modems if a first mode is selected; and providing half-duplex communication between the first and second modems if a second mode is selected.
A system for training modem transceivers for DSL systems under TCM-ISDN interference includes a first modem coupled to a telephone line. The first modem includes an encoder for encoding data. A map switch is coupled to the encoder for selecting one or more bitmaps for encoding based on a mode selected prior to transceiver training. A second modem coupled to the telephone line includes a decoder for decoding data transmitted by the first modem and a map switch coupled to the decoder for selecting one or more bitmaps for decoding based on the mode selected prior to transceiver training.
The present invention provides advantages over the prior art systems and methods by enabling TCM-DSL modems to optimize transceiver training and configuration under TCM-ISDN interference. The optimization includes, for example, adjusting the dynamic range of the modem receiver to accommodate echo signals generated by the modem local transmitter during full-duplex operation, and to adjust the modem transceiver configuration to optimize the usage of available frequency bandwidth.